Mold-clamping mechanisms operating independently of a reciprocacting mold drive, designed to resist the pressure of the injected thermoplastic or other molding material, are well known in the art. These mechanisms are usually operated by a high-pressure fluid, generally a hydraulic liquid, and may comprise a plurality of cascaded pistons whose cylinders are pressurized simultaneously to increase the clamping force, e.g. as described in U.S. Pat. No. 3,847,528.
In order to facilitate the transmission of the piston force to the movable platen, a thrust member rigid with that platen may be clamped between two relatively movable jaws engaed by one of the clamping pistons. It is also possible, e.g. as described in U.S. Pat. No. 3,829,266, to align such a thrust member with an aperture in a support, the thrust member passing through that aperture when the mold is open, and to slide a piston-cylinder unit on the support across the aperture after the thrust member has been withdrawn therefrom in the mold-closed position, the hydraulic expansion of that unit then exerting the desired clamping pressure.
A problem encountered with such fluid-operated clamping mechanisms is the risk that, on account of human error or a malfunction, the mold is not properly inserted between the platens or the force-transmitting devices are not in working position so that the clamping force finds no purchase on the platen and must be absorbed by the cylinder housing. This problem of overtravel is dealt with in prior systems, such as that of the aforementioned U.S. Pat. No. 3,847,528, by the provision of leakage paths draining off the high-pressure fluid whenever the piston stroke exceeds its normal limit. Drawbacks of that solution include wasteful fluid circulation and the instability of the piston position which may give rise to objectionable noises and vibrations.